Detection of metallic objects is usually done with metal detectors that use a transmitting coil to produce an interrogating magnetic field in the environment of a target. This magnetic field induces magnetic responses from the target and surrounding medium, which, in turn, induce a voltage in the receiving coil. This voltage is processed by the metal detector with the intent to remove signals produced by the environment and to differentiate between signals produced by different types of targets.
According to the temporal variation of the magnetic field produced by the electric current in the transmitting coil, the metal detectors are generally classified as pulse-induction (or time domain) and continuous-wave (or frequency domain). The current invention relates to continuous wave (CW) metal detectors,
The main components of a conventional single frequency CW metal detector using analogue circuitry include;
(1) A search head comprising the transmitting and receiving coils in a nulled (induction balanced) arrangement.
(2) A transmitter that generates a sinusoidal signal with constant amplitude applied to the transmitting coil and, at the same time, the timing reference for the synchronous demodulators.
(3) A receiver that amplifies the weak signals from the receiving coil and applies them to the synchronous demodulators.
State of the art single frequency CW metal detectors track the phase angle of the ground signal while synchronously demodulating the received signal into two channels. One channel is continuously maintained in phase with the ground signal and has low gain. The other channel is out of phase by 90° (in quadrature) with the ground signal and thus, approximately in phase with the current in the transmitting coil. This channel has significantly higher gain than the other channel (about 100 times). This allows the detection of relatively weak target signals in the presence of large ground signals. The synchronously demodulated signals are low-pass filtered to remove components at the transmitted frequency and harmonics. The filtered signals are further processed to reject responses due to changes in the environment and to discriminate targets.
It is generally accepted that simultaneous operation on several frequencies would enable this type of CW analogue metal detector to achieve higher performance both in target discrimination and rejection of false signals due to environment. However, the difficulties in the construction of CW analogue multiple frequency metal detectors have prevented their proliferation. This is because for each extra frequency added to a conventional analogue metal detector, a number of blocks (including oscillators, phase locked loops, synchronous demodulators, and low-pass filters) would have to be added. This increases both the cost and the complexity of the detectors.